BR pullet can't stand up

The smallest of my 14 week old pullets can't stand up. I found her yesterday evening after letting everyone out to freerange trying to stand up on her feet with no luck. She has been on a medicated starter. When I pick her up she can kick her legs but not stand on her feet. I saw her take a couple of steps and fall over like the was drunk last night. I isolated her with food and water, and this am she is laying on her side. I fealt her legs/feet and I don't feel any breaks or big swellings. Her feet seem a bit meaty, but normal. From what I read I am thinking it could be Mareks. The pullets have been combined with the layers for 4 weeks. Also, she smells like poo.